In lithographic offset printing, printing plates are typically made of a thin aluminum sheet (the substrate) overlaid with a thin coat of polymer. The polymer is normally photo-sensitive or thermally sensitive. The sensitivity of the polymer is exploited to expose the plate with the desired image using light or laser. Photo-sensitive polymers are exposed with U.V. or visible light whereas thermally sensitive polymers are typically exposed using relatively high powered I.R. lasers. After being exposed imagewise, the plate traditionally requires chemical development before being used on a press. However, recent technologies are "processless" meaning that they require no chemical development or other intermediate steps other than possibly wiping off ablated residue and can be used directly on a press after imaging.
This invention is particularly concerned with processless plates exposed by a laser source, either in a Computer-To-Plate system (C.T.P.) or directly on the printing press (on-press imaging). The polymer coating is either hydrophobic or hydrophilic and changes its water attraction properties with exposure to the laser. Alternatively, the plate can be a dry (or waterless) offset plate. Processless plates exposed on the press may be single use or reusable (i.e. where the same plate is cleaned, re-coated with polymer and used multiple times). (See related application, Ser. No. 08/490,361, now U.S. Pat. No. 5,713,287).
Printing plate performance is typically determined by the "I.R. sensitivity" (i.e. amount of laser power required for imaging) and "imaging resolution" (i.e. the hydrophobic/hydrophilic differential between the irradiated and non-irradiated areas). At odds with plate performance are the requirements for a long shelf-life and robustness. Time combined with the stresses of handling and transport cause the performance of a printing plate to degrade. As such, the performance criteria of I.R. sensitivity and image resolution distinctly determine the required shelf-life and robustness. These conflicting goals are particularly important for processless plates, because it is extremely difficult to create a processless thermal plate combining performance criteria with shelf-life and robustness.
Pre-coated plates, also known as pre-sensitized plates, are manufactured at central locations and distributed to many users. The time between the making and the using of the plate is several months. Accordingly, the chemicals used to coat the plates must be sufficiently robust and durable to withstand transport and handling, and to endure a relatively long shelf-life (typically on the scale of twelve months). In order to achieve such a shelf-life, the chemicals employed on pre-coated plates must be relatively stable and non-reactive. As such, pre-coated plates sacrifice performance, particularly I.R. sensitivity, for robustness and shelf-life. If plates could be coated on-site at the printing plant, then this sacrifice could be effectively eliminated because the required shelf-life would only be the duration of the printing run (typically on the scale of hours or days) and there would be relatively little handling and transportation involved. Consequently, more aggressive coating chemicals, which tend to react with one another, may be used to create plates with superior I.R. sensitivity and imaging resolution at the expense of diminished shelf-life and robustness.
On-site coating has been used in the printing industry before. The best known examples are the "Wipe-On" plates in which a photo-sensitive composition is spread on the aluminum plate by hand or machine. The coated plate is then treated by mechanical, chemical or electrical processes to improve the hydrophilic properties and adhesion of the polymer coating. "Wipe-On" plates are used less frequently now, because their chemistry is the same as that of pre-coated plates, but their performance is inferior; their only advantage is lower cost. In this invention a different chemistry is used for on-site coating which cannot be used on pre-coated plates due to the strong reactive nature of the chemicals involved. The high degree of chemical activity generates plates superior to pre-coated plates at the expense of shelf-life and robustness which are of reduced importance for on-site coating.
European patent EP-0-652-483-A1, hereinafter referred to as Ellis, discloses a heat sensitive coating for use on a pre-coated processless lithographic plate. The coating consists of three chemicals:
(i) a hydrophobic polymer which reacts under the action of heat and/or acid to become hydrophilic (Page 2, lines 51-52); PA1 (ii) a photo-thermal converter which is capable of absorbing I.R. radiation and converting it to heat and/or acid (Page 2, lines 43-44 and Page 4, lines 35-36); PA1 (iii) a thermal acid generator which releases acid under the action of heat generated by irradiation of the photo-thermal converter (Page 5, lines 22-24).
Ellis does not teach direct addition of acid to the mixture because the Ellis invention is concerned about the detrimental effects on the shelf-life and robustness of the coating which are essential requirements of a pre-coated plate.